p.s. does the palette have to be forced in black and white?
This is what I like to call a "wow, I had a lot of colors" moment.
Ifranview will occasionally stop itself from reducing a picture to 16 colors or below to prevent the image from becoming horrible quality.
This is a easy fix, but it requires that you do all the work. Just simply open up your image in a paint program and try your best to color over some of the lesser used colors with another color that's used a lot in the tiles.
And when you index the image in photoshop, in the Forced Box just set it to None.